This invention relates to an improved seismic energy source used to create pulses or shock waves in a liquid medium such as water. More particularly, it relates to a simple gas-powered apparatus which is easily operated, more reliable than those of the prior art, and whose signature and frequency spectrum can easily be modified.
In prospecting in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating sonic pulses or shock waves into the water. Since water is a good conductor of sound, it normally is not necessary to generate pulses near the floor of the waterbody; they can be, and desirably are, produced near the water's surface. These pulses propagate down through the water, across the waterfloor interface, into the subfloor geologic formations and are, to some extent, reflected back across the same path to an array of geophones, or like equipment, waiting near the surface of the water. Analysis of the signals produced by the geophones can provide some instruction concerning the structure of the subfloor geological formation and attendant petroleum accumulation in those formations.
The term "water" as used herein is meant to include swampwater, mud, marshwater and any other liquid containing sufficient water to enable operation of the invention.
There are many ways of generating a sonic pulse in a liquid. For instance, explosives introduce strong pulses into the water and accordingly achieve substantial penetration into subfloor formations. Certain obvious drawbacks exist in their use: they are dangerous to store, handle, and use. When used in open water they kill marine life. In crowded areas such as harbors, explosives cannot be used at all. Explosives are orders of magnitude more expensive to use, on a per-shot basis, than are gas guns. Modification of the explosive source's sonic signature to achieve an acceptable spectrum distribution is difficult.
Another method of generating a sonic pulse is by discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing gaseous bubble. However, the efficiency of this method is quite low in that only a few percent of the energy charged to the capacitors is found in the shock wave produced on discharge.
Apparatus using explosive gas mixtures, e.g., propane and air, to produce the sonic pulse have gained wide acceptance. The two major types of explosive gas guns are those which operate by exploding a gas mixture behind a flexible membrane which in turn is in contact with the water and those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Pat. No. 3,658,149; an example of the latter can be found in U.S. Pat. No. 4,193,472.
Open guns using high pressure compressed gases, instead of an explosive mixture, have achieved a wide acceptance in the industry. Typical designs for open-ported compressed gas guns are found in U.S. Pat. No. 3,653,460 to Chelminski and U.S. Pat. No. 4,141,431 to Baird. These guns employ two pressurized chambers, i.e., a control chamber and a gas holding chamber, which are sealed by a spool-shaped valve or shuttle. The gun is fired by abruptly releasing gas from the control chamber. The gas in the gasholding chamber forces the shuttle into the control chamber thereby simultaneously exposing exhaust ports. These ports allow the gas stored in the gas holding chamber to explosively exit into the water. The control chamber is then re-pressurized, thereby moving the shuttle back into a position sealing the gasholding chamber. The gun is again ready to "fire".
Guns employing this design have certain liabilities which are quite difficult to correct. The period during which the exhaust ports are open after escape of the initial burst of gas is one in which no useful operation is performed. The initial burst of gas through the exhaust port is the one which produces the useful portion of the shock wave. Obviously the gas lost from the exhaust ports during the repositioning of the shuttle is wasted. The apparatus of the invention disclosed herein consumes a significantly smaller amount of compressed gas than do those of the prior art which have a shuttle which must reverse itself prior to firing. The mechanical stress on the shuttle of the invention is much less than on the reversing shuttle of the prior art.
A compressed gas gun eliminating several problems associated with prior guns is disclosed in U.S. Pat. No. 4,180,139. This patent discloses a gun having a single cylindrical gas chamber with central exhaust ports about its periphery. Inside the gas chamber resides a moveable shuttle also having ports about its center. When the shuttle is moved from one end of the gas chamber to the other end, via the action of an integrated actuator, the ports in the shuttle momentarily align with those in the gas chamber wall and allow an amount of compressed gas to escape. Once the shuttle reaches the other end, the gun is in position to "fire" again awaiting only the build-up of pressure in the gas chamber and actuating mechanism. Although this device has a number of advantages over those of the prior art, e.g., efficient compressed gas usage, the geometry of the device precludes any reasonably facile alteration in the frequency and spectrum distribution of the sonic pulse it produces. The two physical dimensions of the gun, i.e., exhuast port geometry and gas chamber size, defining the signature of the sonic pulse cannot be significantly changed.
In contrast, the present invention is designed not only to eliminate valving which must reverse itself prior to repressuring and firing, but also to provide a ready modification of the sonic signature should such be desired.